Flip chip microelectronic assembly is well-established in the semiconductor industry. The assembly process involves the direct electrical connection of face-down electronic components onto substrates, circuit boards or carriers by means of conductive bumps on the chip bond pads. In contrast, older technology involves wire bonding of each bond pad of face-up chips to substrates, circuit boards or carriers. The advantages of flip chip packaging over older technology include smaller size, better performance, increased flexibility, reliability and lower cost. For example, eliminating packages and bond wires reduces the required board area by up to 95%, and requires far less height. Weight can be less than 5% of packaged device weight. In fact, the flip chip can be even smaller than Chip Scale Packages (CSP) because its size is the size of a chip. Flip chip materials are also becoming more widely available, further lowering production costs.
A conductive bump serves several functions in flip chip assembly. Electrically, the bump provides a conductive path from chip to substrate. The bump also provides a thermally conductive path to carry heat from the chip to the substrate. In addition, the bump provides part of the mechanical mounting of the chip to the substrate. Finally, the bump provides a spacer, preventing electrical contact between the chip and substrate conductors, and acting as a short lead to relieve mechanical strain between chip and substrate.
One method of forming bumps on semiconductor wafers comprises a process known as stud bumping, which creates bumps on wafer chips by a wire bonding technique that is modified from the older wire bonding technology. This technique makes a ball for wire bonding by melting the end of a wire to form a sphere. The ball is attached to the chip bond pad as the first part of a wire bond. To form bumps instead of wire bonds, wire bonders are modified to break off the wire after attaching the ball to the chip bond pad. The ball, or “stud bump” remaining on the bond pad provides a permanent connection to the underlying metal on the chip. Traditionally, gold wire is used in stud bumping. However, copper wire, which is relatively cheaper, is gaining wider acceptance in the industry.
In current stud bumping systems, a semiconductor wafer is typically placed onto a top plate or heating plate of a vacuum chuck, and held directly on the heating plate using vacuum suction. The heating plate may be made of a variety of material, such as porous ceramic, or anodized or nickel plated aluminum. A plurality of vacuum holes are included on the surface of the heating plate to exert vacuum suction force for securing the wafer on the heating plate. Thereafter, gold or copper bumps are bonded or bumped onto the heated semiconductor wafer.
Traditionally, the heating plate for heating the wafer is made rigid and non-compliant in order to provide adequate support for the wafer during bonding. One reason is that compliant material is generally more vulnerable to heat damage as compared to, say a metallic support, which might put the wafer at risk of damage during bonding. Another reason is that if the surface is compliant or resilient, it may be more difficult to control the tolerances of the bonding process and may for instance result in damage due to excessive flexion of the semiconductor device during bonding due to flexibility of the underlying support layer.
However, it was discovered that the traditional approach of implementing a rigid mounting support for stud bumping of a semiconductor wafer has a number of problems. One problem is that whilst the heating plate has to be machined to be as flat as possible, there are often differences between the contours of the surface of the semiconductor wafer and the contours of the heating plate that are in contact. Such unevenness in the surface of the heating plate or non-conformity in contours of the surfaces sometimes leads to vacuum leakage, which diminishes the vacuum suction force securing the wafer to the heating plate. Cratering or peeling of a bond pad on the semiconductor wafer has been known to occur through direct contact of the wafer on the heating plate surface. Moreover, in some types of semiconductor wafers, it was also found that the back surface of the semiconductor wafer in contact with the heating plate had a propensity to peel off after the stud bumping process. For the above reasons, there is a need to mitigate the shortcomings of utilizing rigid mounting supports.